Tensioned metastable fluid radiation detectors are known However, current detectors have a higher amount of false positive detection than is desirable. New methods are needed that can be used to reduce the detection of false positives.
Nuclear radiation sensors are needed that possess over 90% intrinsic efficiency, the ability to determine in near real-time the direction of the source of radiation to within 10 degrees for a wide range of nuclear materials including spontaneous fission neutrons, gammas as well as alpha emissions, with radiation energies ranging from sub-eV to MeV. They should have the ability to remain blind to photon and cosmic interference, the ability to ascertain the characteristic multiplicity of WMD emission signals, the ability to turn on and off within microseconds so as to be feasible to operate in pulsed interrogation environments without becoming saturated. Further, they should have the ability to function in harsh environments with low false-positive detection events, the ability to selectively identify shielded or unshielded WMDs from their neutron-alpha signatures compared with state-of-the-art systems. Only 8 kg of Pu is considered to constitute a “significant” quantity, i.e., a level constituting a threat of nuclear terrorism. Consequently, the WMD threat no only requires one to be able to detect about 8 kg of Pu but also, to ensure that Pu and other SNMs special nuclear materials (like U) are not diverted.
Unfortunately, present-day radiation sensor systems as employed for combating Nuclear Terrorism are largely adaptations of systems developed for nuclear power usage for over 50 years, and do not enable a single system to respond to these needs.
Known TMFD sensor technology is based on placing ordinary fluids such as water or acetone in thermodynamic states of “tension” (not superheat) metastability under sub-vacuum conditions at room temperature. This is analogous to stretching a rubber band: the more the tension, the less is the energy required to snap the intermolecular bonds holding the material together. Thereafter, excess energy deposited from the direct strike of a nuclear particle (e.g., keV to Mev fission neutron or alpha recoil) onto a tensioned metastable fluid results in the nucleation of nanoscale (50-100 nm) critical sized bubbles [rc=2σ(Pv−Pext]; where, σ is the surface tension, Pv is the vapor pressure inside the cavity and Pext is the external liquid pressure] which grow to visible (mm) size and then implode back to the liquid state accompanied by audible shock signals and light flashes which can be recorded using conventional electronics. The amount of deposited energy within a dimension commensurate with formation of a superheated vapor bubble of twice the critical radius “rc” is a function of the LET of the radiation, fluid properties and Pneg.
Pneg states in the −1 bar to −10 bar range suffice for reliable, gamma-beta blind detection and even spectroscopy of neutrons from the eV to the MeV+ range and for detecting alpha recoils and fission fragments when using TMFD sensor fluids such as DFP (C5H2F12), PFO (C8F18), acetone, R-113 (C2Cl2F3). TMB (C2H9BO3), methanol. Fluids such as DFP, PFO, acetone enable high efficiency (95%+ theoretical intrinsic efficiency) fast neutron detection in the fast (above keV energy), whereas, borated and/or Cl/Fl atom inclusive fluids enable neutron absorption (n,alpha and n,p leading to bubble formation) based detection of fast and thermal (sub-eV) energy neutrons. Alpha recoil and fission fragment detection for dissolved radionuclides is enabled with over 95% intrinsic efficiency with spectroscopy for all TMFD fluid types.
The Neutron Radiation (Gamma/Beta Blind) Detection Principle of TMFDs is well known. TMFDs are radically different from bubble chambers (Glaser, 1958) or superheated drop detectors (SDDs) which operate in the superheat (i.e., above boiling points) regime, and, for which the efficiency of detection of neutrons is limited by the sensitive volume of droplet suspensions which is typically 1/1000 that for TMFDs and furthermore require minutes to hours for reset and cannot detect alpha particles or fission fragments as for TMFDs. Such attributes make TMFDs unique compared to state of the art sensors such as 3He & BF3, (which depend on charge collection), or scintillation (e.g., NE-213 or fluor cocktail) systems.
dE/dx (MeV/cm) for ion moving through Ion (1 MeV)example TMFDe (electron)-photon1H (proton; z = 1)183B (boron; z = 5)3,768C (carbon; z = 6)4,217O ( oxygen; z = 8)4,455
As can be seen, for a typical energy level of about 1 MeV associated with fission neutrons, the LET of electrons is at least 100 to 1.000 times lower than that from heavy ions, including protons, alpha particles, B, C, O, and the like. MeV gamma photons lose energy primarily via Compton scattering with electrons, and can at most deliver about 0.88 MeV even in a direct knock-on incident. The LET of 1 MeV photons interacting with C, H, O, F, Cl atoms in TMFD fluid molecules will also be in the 1 MeV/cm range. We have found that, for Pneg levels of about −2 bar through −10 bar (the threshold for detection in isopentane, the energy deposited within “rc of about 50 nm” required for recoil detection is in the range of about 10 to 100 keV. A recoiling 1 MeV carbon ion would readily be able to deposit the required about 40 keV and hence, permit detection. At Pneg of about −10 bar, even a 1 MeV recoil proton is capable of delivering the equivalent of a 250 eV carbon recoil which is sufficient for detection. However, due to the 1,000-fold lower LET for electrons and photons, it is impossible to deposit enough energy within 2×rc length scales and as a consequence. TMFD neutron sensors must be designed to offer gamma-beta blindness even in about 10+ R/h fields. This will provide the possibility for detecting unique active photon interrogation of shielded nuclear materials.
Improved devices for reliable detection of fast (1 to 14 MeV) and thermal (eV range) neutrons in the presence of a continuous source of gamma rays (about 0.67 MeV from 137Cs, about 1.2 MeV from 60Co, about 4.4 MeV from Pu—Be and up to about MeV from 252Cf are needed. Detection must be possible in neutron fields of up to about 6-9 MeV from X-Ray sources are needed. Detection must be possible in neutron fields of up to 1011 n/s and photon emissions of over 1011 gammas's (about 5-10 R/h and even higher for X-ray interrogation fields). The first sensor system shown in FIG. 1 is based on the centrifugal force principle (also called Centrifugal Tensioned Metastable Fluid Detector or CTMFD).